MIAMI -- Lake Okeechobee was expected to drop this week to its lowest level on record, threatening a key source of water for nearly 5 million people and the Everglades during South Florida's worst known drought.

The nation's second-largest freshwater lake held at 9 feet on Tuesday, less than a half-inch above the previous record of 8.97 feet, set May 24, 2001, after another long drought. The average water level for this time of year should be around 13 feet.

The summer-through-fall rainy season has started in parts of the region, but only above-average rainfall would help replenish the lake, officials said.

"If we have below-average or even average rainfall, we could come out of rainy season and still be in a drought," said Carol Wehle, executive director of the South Florida Water Management District.

Lake Okeechobee is a backup drinking water source for millions of people in South Florida and the lifeblood of the Everglades. The region is largely dependent on the lake during dry periods, when it can be used as a reservoir.

The 18-month drought already has led to severe water restrictions for homes and businesses across the state. Limits have been placed on watering lawns and washing cars. Golf courses are speckled and brown because of limits on water use. Citrus growers and other farmers also have been ordered to cut their water use by half.

Only isolated showers were forecast over the lake until the weekend, when more substantial rain was expected, according to the National Weather Service. Forecasters said any rain would only stabilize its levels temporarily because more rain was still needed farther north in the Kissimmee River valley, which feeds the lake.

The last time the lake rose was after Tropical Storm Ernesto brushed past Florida in August, according to the Jacksonville district of the Army Corps of Engineers. The lake level rose 1.5 feet to 13.5 feet after that storm, said Andrew Geller, a corps hydraulic engineer.

"That was the end of our significant rainfall," Geller said.

Bernie Ortega, owner of Bernie's Garden Center in Miami, said his business has suffered since the drought began. He hopes customers learn how to care for their lawns with limited water supplies.

"It's really a problem that you have to consider for many, many years to come," Ortega said. "Because water restrictions are going to be with us for a long time. As more and more people move in, water is going to become more of a luxury."